The effects of water regime on phosphorus responses of rainfed lowland rice cultivars.
OBJECTIVE
Soil phosphorus (P) solubility declines sharply when a flooded soil drains, and an important component of rice (Oryza sativa) adaptation to rainfed lowland environments is the ability to absorb and utilize P under such conditions. The aim of this study was to test the hypothesis that rice cultivars differ in their P responses between water regimes because P uptake mechanisms differ.


METHODS
Six lowland rice cultivars (three considered tolerant of low P soils, three sensitive) were grown in a factorial experiment with three water regimes (flooded, moist and flooded-then-moist) and four soil P levels, and growth and P uptake were measured. Small volumes of soil were used to maximize inter-root competition and uptake per unit root surface. The results were compared with the predictions of a model allowing for the effects of water regime on P solubility and diffusion.


RESULTS
The plants were P stressed but not water stressed in all the water regimes at all P levels except the higher P additions in the flooded soil. The cultivar rankings scarcely differed between the water regimes and P additions. In all the treatments, the soil P concentrations required to explain the measured uptake were several times the concentration of freely available P in the soil.


CONCLUSIONS
The cultivar rankings were driven more by differences in growth habit than specific P uptake mechanisms, so the hypothesis cannot be corroborated with these data. Evidently all the plants could tap sparingly soluble forms of P by releasing a solubilizing agent or producing a greater root length than measured, or both. However, any cultivar differences in this were not apparent in greater net P uptake, possibly because the restricted rooting volume meant that additional P uptake could not be converted into new root growth to explore new soil volumes.
Huguenin-Elie
O
O
International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines.
Kirk
G J D
GJ
Frossard
E
E
D018533:Biomass; D005308:Fertilizers; D008954:Models, Biological; D012275:Oryza / Q000145:classification* / Q000254:growth & development / Q000378:metabolism*; D010758:Phosphorus / Q000378:metabolism*; D018517:Plant Roots / Q000502:physiology; D018520:Plant Shoots / Q000502:physiology; D011891:Rain; D012987:Soil; D013997:Time Factors; D014867:Water / Q000502:physiology*
D005308:Fertilizers; D012987:Soil; D014867:Water; D010758:Phosphorus